The statements in this section merely provide background information related to the disclosure and may not constitute prior art.
In the oil and gas drilling and production industry, viscous aqueous fluids are commonly used in treating subterranean wells, as well as carrier fluids. Such fluids may be used as fracturing fluids, acidizing fluids, and high-density completion fluids. In an operation known as well fracturing, such fluids are used to initiate and propagate underground fractures for increasing petroleum productivity.
Viscous fluids, such as gels, are typically an aqueous solution of a polymer material. A common continuous method used to prepare viscous fluids at a wellbore site, involves the use of initial slurry of the polymer material in a hydrocarbon carrier fluid (i.e. diesel fluid) which facilitates the polymer dispersion and slurry mixing. Although this process achieves the required gel quality, the presence of hydrocarbon fluids is often objected to in particular fields, even though the hydrocarbon represents a relatively small amount of the total fracturing gel once mixed with water. Also, there are environmental problems associated with the clean-up and disposal of both hydrocarbon-based concentrates and well treatment gels containing hydrocarbons; as well as with the clean-up of the tanks, piping, and other handling equipment which have been contaminated by the hydrocarbon-based gel.
Other applications used for the continuous mixing of viscous treatment gels include gelling the polymer in a hydrocarbon carrier that is mixed with water to produce the fracturing gel which is then flowed through baffled tanks providing first-in/first-out (FIFO) flow pattern, and allowing for the hydration time of the gel. Yet another technique for mixing of dry polymer directly to produce viscous treatment gels is described in Allen, U.S. Pat. No. 5,426,137, Allen, U.S. Pat. No. 5,382,411, and Harms et al., U.S. Pat. No. 5,190,374. These techniques, while potentially effective, require several complicated steps to prepare the gel, which presents drawbacks in an oilwell setting. Further, U.S. Patent Application 2004/0256106 A1 discloses an apparatus without an eductor, for substantially hydrating a gel particulate using a mixer in conjunction with an impeller located within the mixer housing, which prevents formation of gel balls.
Other techniques and equipment useful for the continuous mixing of viscous treatment gels without utilizing a hydratable polymer in a hydrocarbon are described in Pessin et al., U.S. Pat. No. 7,866,881, which discloses preparation of a viscous treatment gel from dry polymer utilizing a constant volume educator and mixing chimney, where the eductor operates at a constant water rate and pressure thus producing a concentrated polymer slurry. While effective in preparing an aqueous slurry from dry hydratable polymer and water, there still exists need to further minimize equipment size, space requirements, and efficiency.
Some hydration tanks configured in a first-in/first-out configuration are vented tanks which operate by use of gravity to flow a hydrating gel, formed of a polymeric viscosifier in aqueous solution, through the tank. As the polymer concentration in the gel increases, viscosity increases, and gravity flow of the gel is only possible up to a practical polymer concentration. As a result such systems are not useful to handle hydration of gels having a high concentration of viscosifier.
Therefore, there is a need for apparatus and methods useful for hydrating constituents at high concentrations to prepare viscous treatment gels in a continuous mode, without the use of hydrocarbon carriers, and with decreased equipment size and space requirements, such need met, at least in part, by the following disclosure.